1. Field of the Invention
This invention relates to a vertical band saw machine, more particularly to a vertical band saw machine with a speed ratio varying device for a transmitting shaft which is driven by a motor so as to effect rotational movement of an endless band saw blade.
2. Description of the Related Art
Referring to FIG. 1, a conventional vertical band saw machine 10 is shown to include a lower support 12 with a worktable 16 for supporting a workpiece (not shown), a coupling unit 11 which includes a lower coupler 111 secured to the lower support 12, and an upper coupler 112 adjustably coupled with the lower coupler 111 by means of a screw fastener, an upper support 13 secured to the upper coupler 112, upper and lower blade mounting pulleys 15,14 rotatably mounted on the upper and lower supports 13,12, and an endless band saw blade 17 trained on the pulleys 15,14 and passing through the worktable 16. When a motor (not shown) is actuated, the lower blade mounting pulley 14 is driven to rotate the band saw blade 17 for sawing the workpiece on the worktable 16. Since the rotational speed of the lower blade mounting pulley 14 is fixed, the sawing speed of the band saw blade 17 remains unchanged when sawing workpieces of different thicknesses. Thus, the service life of the band saw blade 17 may be shortened, and the sawing quality of the workpiece may be adversely affected.